HAIR STRUCTURE OF THE MONOTREMATA 481 
flattened and more wavy type (fig. 45). The first two are simi- 
lar to the spiny and wavy hairs already described as occurring 
on the dorsum. 
The covering of the tail is similar to that of the dorsum except 
that the spines are a trifle more slender and acuminate (figs. 60 
and 61). About the periphery of this caudal tuft of spines oc- 
curs a zone or border of smaller spines which are unpigmented, 
and are much lighter in color than any of the spines elsewhere 
upon the body. Whenever the spines of the tail are elevated, 
these lighter marginal spines obscure the darker ones in the 
center of the tuft and cause the tail to appear a light creamy 
white color, and consequently to be very conspicuous when 
viewed from behind. 
Upon the head occur short, light colored spines (fig. 62), short 
spiny flattened hairs (figs. 63 to 65), and short wavy hairs (fig. 
66). All of these are similar to the spines and hairs of the dor- 
sum, but shorter. 
The flanks are covered by small curved spines (fig. 50) and long 
spiny hairs (fig. 49) which appear to be transitional between the 
spiny hairs of the venter and the curved spines of the flanks just 
mentioned. 
The feet bear long spiny flattened hairs and shorter flattened 
wavy hairs as does the venter, though these of the latter type 
are few in number or absent altogether. 
A comparison of figures 45 to 53 will give a notion of the nature 
of the transition from the finest wavy hair of the body to the most 
robust of the spines which are found in the hip tufts. 
The wavy hair with its thickness slightly increased becomes 
the flattened straighter type shown in figures 47 and 48, and this 
becoming still thicker, especially in its medial portion, gives rise 
to the spines of the type illustrated by figure 50. A progressive 
increase in length and thickness of such a spine produces those of 
the types represented by figures 51 to 53, successively. 
Thus a comparison of the gross characteristics of the hairs and 
spines suggests a transition in form, and an examination of their 
microscopic structure lends support to such a supposition. 
